1. Field of the Invention
The present invention relates to sensors and particularly to a phase noise cancellation system for an interferometric fiber optic sensor arrangement.
2. Description of the Prior Art
Interferometric fiber optic sensors are highly sensitive devices for the measurement of weak time-varying measurand fields, such as acoustic pressure, vibration and magnetic fields. In order to achieve remote passive optical interrogation of fiber interferometers, a number of demodulation techniques based on phase generated carrier (PGC) concepts have been developed. These techniques utilize an unbalanced interferometer and laser frequency modulation to induce high-frequency phase shift `carriers` in the interference output of the sensor, which are encoded by lower-frequency `signal` phase shift information of interest. Both passive (sine/cosine) homodyne and synthetic heterodyne techniques are possible. One problem which arises when using this form of demodulation approach is that of laser frequency-jitter induced phase noise. Due to the required use of an unbalanced interferometer for operation of the PGC demodulation, the interferometer is also inherently susceptible to laser frequency jitter (laser phase noise). This problem is well understood and characterized for a number of commonly used diode lasers.
In the standard prior art, phase generated carrier (PGC), passive homodyne system, signal dependent on the sine and cosine of the phase of an unbalanced sensor interferometer are fed to a differentiate and cross multiply demodulator which provides an output signal that contains both the time-varying phase difference between the arms of the sensor interferometer and phase noise due to laser emission frequency jitter. This phase noise typically limits the detection sensitivity of the prior art sensing system.
The emission frequency of a laser can be stabilized by locking it to a reference cavity, such as a Fabry-Perot interferometer. Such locking of the emission frequency of a laser can be used to greatly reduce low frequency phase noise. However, this technique can only be used with unmodulated lasers. To date, there is no prior art technique, known to applicant, for achieving phase noise reduction of a frequency modulated laser in a fiber optic interferometer system.